1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected not to retain title.
2. Technical Field
The invention is related to submillimeter wave generators and in particular to a submillimeter wave generator which does not depend upon non-linear semiconductor processes to produce power, but rather employs optically-controlled electronic switches connected to a voltage source providing the submillimeter radiation power.
3. Background Art
There are no efficient submillimeter wave sources operating in the 300 to 3000 GHz region which are lightweight and produce on the order of a milliwatt power, despite current efforts to find such a source. The need for efficient local oscillators for heterodyne mixers in the 300 to 3000 GHz region is critical because of the potential applications in space-based submillimeter wave imaging radar, non-destructive testing, plasma diagnostics, weapon and contraband detection and communications. Such local oscillators must have reasonable power (in the range of milliwatts up to watts) and are required to cover a wide spectral range. The art has made numerous unsuccessful attempts to meet this need, but so far there appears to be little prospect of succeeding, as will now be described.
Lasers developed for this purpose are (individually) restricted to essentially one single wavelength. Some tunability can be achieved by optical techniques, but only over very limited bandwidth.
Microwave generators capable of generating submillimeter waves, such as carcinotrons, do not operate efficiently at wavelengths shorter than one millimeter and are excessively heavy, consume considerable power and have short lifetimes, making them relatively unsuitable to use in space flight.
Available solid state oscillators, such as GaAs Gunn diodes and IMPATT diodes are highly efficient and tunable but are limited to frequencies up to about 75 and 125 GHz, respectively, because of fundamental limitations on maximum current density in such devices. Recent research on quantum well negative differential resistance devices or oscillators has produced results characterized by very low power (less than microwatts) at much higher frequencies than the solid state oscillators discussed above. Higher frequencies (up to 500 GHz) have been achieved also by generating harmonics of the oscillator frequency using solid state nonlinear devices such as GaAs varactors or varistors. This approach is limited by reduced power output at the higher frequencies, typically less than microwatts. As disclosed in U.S. Pat. No. 4,954,864 by Joseph Maserjian, one of the inventors herein, some attempts have been made to partially overcome this limitation using large arrays of such devices. However, the highest frequencies achieved in such quantum well devices are on the order of only 500 GHz because of fundamental limitations.
Thus, it has not seemed possible to provide a submillimeter wave source operating in the 1000 to 3000 GHz frequency range with power output on the order of milliwatts.
It is therefore an object of the invention to produce submillimeter waves without relying upon nonlinear semiconductor devices such as PN junctions and quantum wells to provide the power, so as to escape the fundamental constraints which so far have held submillimeter power output at 500 GHz to sub-microwatt levels.
It is a further object of the invention to provide on a single semiconductor substrate a submillimeter wave source capable of producing submillimeter waves with at least one milli-watt power at frequencies between 300 GHz and 3000 GHz.
It is another object of the invention to provide a submillimeter wave source meeting the foregoing objectives and further having a tunable submillimeter wave output frequency.
These and other objects and advantages of the invention will become apparent in the following detailed description when taken in conjunction with the accompanying drawings.